Ease of Detachment of a Space Helmet

Question

How easy is it to disconnect and remove the helmet of a space suit? More specifically...

• How long does it take for one to remove his own helmet?
• Would it be feasible for somebody to remove another's helmet in a relatively quick manner?
• Would it be possible for somebody in vacuum to reattach his own helmet if it were removed?

I am aware that there are a variety of space suits, and I am under the impression that some EVA suits are more of a hard shell without a separately removable helmet. On that point, I welcome answers relating to any and every variety of suit you are familiar with.

Answer 1

For the Orlan spacesuits this is not possible. From Encyclopedia Astronautica

It consisted of flexible limbs attached to a one-piece rigid body / helmet unit. The suit was entered through a hatch in the rear of the torso.

The helmets of the Sokol suits are easy to open, like shown in this video: (3:15)

The Strizh suit seems to have the same mechanism.

Answer 2

For the Shuttle / ISS Extravehicular Mobility Unit, it is not too difficult:

The connecting ring of the helmet is similar to the rings used for the body-seal closure. Mobility is not needed in this ring because the inside of the helmet is large enough for the crew member’s head to move around. To open or lock any of the connecting rings, one or two sliding , rectangular-shaped knobs are moved to the right or the left. When opened, the two halves of the connecting rings come apart easily. To close and lock, one of the rings slides part way into the other against an O-ring seal.The knob is moved to the right, and small pins inside the outer ring protrude into a groove around the inside ring, thereby holding the two together . All suit openings have locking provisions that require a minimum of three independent motions to open.This feature prevents any accidental opening of suit connections.

From here.